The present invention relates to a "reach-in" merchandiser type of refrigerated display case or cabinet used primarily in retail food and supermarket outlets. The term "refrigerated", in accordance with the present invention is intended to incorporate those cases maintained at a temperature at or in excess of 32.degree. F., such as display case utilized for display of milk and fresh foods, and those cases maintained below 32.degree. F., such as frozen food cases. In addition, references are made herein to the use of transparent doors, since those are the types of doors most frequently utilized in such retail outlets. Other types of doors could also be employed within the scope of the present invention.
An increasing market demand made by managers of retail food outlets is to reduce the energy consumption of refrigerated display cabinets in order to reduce operating costs wherever possible. Various approaches have been used to conserve the energy required to operate these display cases. When the display case is operated as an open-front or an open-top cabinet to permit freer customer access to the stored products, the primary refrigerated air band circulated within the cabinet is often protected by one or more outer air bands such as disclosed in U.S. Pat. No. 4,144,720 issued to Subera et al which is assigned to the same assignee as the present application. In the Subera patent, an open-front refrigerated display case having primary and secondary air conduits is disclosed. An additional ambient air curtain is provided so that the refrigerated air band is protected by two outer air bands flowing in the same direction across the open access area of the case. Another approach to conserve operating energy has been to install barrier doors in order to limit the contact of ambient air with the refrigerated air band to those time periods during which customers or employees hold open the barrier doors. This type of refrigerated display cabinet is shown in U.S. Pat. No. 4,072,488 to Johnston and in the above referred to copending applications of the inventor hereof.
It has not been deemed necessary to protect the refrigerated air band by the provision of additional outer air bands to reduce operating energy and to limit exposure to ambient air during those time periods when the barrier door is opened. Consequently, the prior art refrigerated display cabinet having barrier doors have been constructed for single air band operation, whereas the open-front and open-top refrigerated cases have been constructed with multiple bands in order to protect the refrigerated air band from contact with ambient air.
This invention is based on recognition that a more energy efficient refrigerated display case can be constructed by utilizing both a barrier door and multiple air bands to reduce inward heat transfer from the warmer ambient air. If these divergent solutions to the problems of energy conservation are utilized in a unique manner, operating costs can be lowered due to the energy saved.
In the operation of all types of refrigerated display cabinets, it is desirable to include a system for automatically defrosting the refrigeration coils. The defrost cycle can be actuated either at set periodic time intervals or when the frost buildup within the system has reached a certain predetermined level. Such systems are typically thermostatically controlled so as to switch from a refrigeration cycle to a defrost cycle of operation. In this manner of operation it is possible to avoid any significant frost buildup within the display cabinet such that inoperability and spoilage of food products would occur.
There have been three different approaches for defrosting refrigerated display cabinets in the art. These are, utilizing electric resistance heaters; passing a compressed refrigerant gas having a high specific heat through the refrigeration coils; and, circulating ambient air through an air conduit in which the refrigeration coils are positioned. Due to the increased cost of energy, efforts have been made to place more emphasis on the utilization of ambient air defrost system as an alternative to the electrical resistance heaters or compressed refrigerant gas defrost systems.
The present invention based on using both a barrier door and multiple air bands in a refrigerated display cabinet is usable with each of these three different defrosting approaches. In particular, the energy conserving refrigerated display case of the present invention provides for a defrost cycle in which ambient air is circulated through an air conduit in which the refrigeration coils are positioned in order to achieve lower operating costs. One energy efficient way to defrost the coils with ambient air is to connect the air conduit with ambient air ports covered by openable gates which permit the through-flow of air when in an open position. Another energy efficient way to defrost is to create a gap between the barrier door and its associated access opening and to use this as a port in order to provide for through-flow of ambient air in order to defrost the refrigeration coils.
In refrigerated cabinets having single refrigerated air bands protected by glass barrier doors, the refrigerated air band may have a temperature as low as about -25.degree. F. and a humidity ratio of 0.0002 lbs water/lb. dry air. Upon opening the barrier door to gain access to the displayed products, this air band comes in contact with ambient air which can have a temperature as high as about 75.degree. F. and a 55% relative humidity which involves a humidity ratio of 0.01 lbs. water/lb. dry air. The temperature difference upon opening the barrier door is then about 100.degree. F. and the humidity ratio difference is approximately 0.0098 lb. water/lb. dry air. This type of contact can result in considerable heat transfer to the refrigerated air band both in the form of heat exchange between the moving air band and the stationary ambient air and by means of mass transfer of moisture which increases the refrigeration energy required for operation of the display cabinet since that moisture must be condensed in the form of frost or ice. The heat of condensation for that moisture must be extracted by the refrigeration apparatus. This type of heat transfer can raise the refrigeration load in a cabinet even above that required by multi-air band open-front cases having no barrier doors. To solve this problem, one or more outer air bands can be provided to protect the primary refrigerated air band against direct contact with ambient air during those times when the merchandiser doors are opened. Also the circulation of the outer air band or bands reduces heat transfer from outside of the cabinet because this secondary air band will have a temperature approximately 10.degree. F. to 15.degree. F. higher than that of the primary air band and hence the outer surfaces of the cabinet will also be at higher temperatures than when only a primary air band is used.
The problem of protecting refrigerated air bands against contact with the ambient air has been solved in other ways using curtains which do not permit for the low energy air defrost systems or do not permit customer entry. Beckwith et al, U.S. Pat. No. 3,403,525 discloses a night curtain which is to be placed over the normally open access area of a refrigerated case in order to reduce energy consumption during the "nonsales" hours, but with this arrangement no air defrost or customer entry is possible. Vogel, U.S. Pat. No. 4,117,698 discloses a retractable night curtain for use during closed store hours during which no provision is made for customer entry.
In designing refrigerated display cabinets to be defrosted by ambient air, a number of different systems have been employed. One type of system which employs ambient air during a defrost cycle is exemplified by U.S. Pat. Nos. 3,403,525; 3,850,003 and 3,937,033, all to Beckwith et al. These systems use fans separate and distinct from the main circulating fans to move ambient air across the refrigeration coils for defrosting. The additional fans are turned on only during the defrost cycle of operation for pulling ambient air from outside of the display cabinet directly into the air conduits located within the walls of the cabinet. A second type of system is illustrated in U.S. Pat. No. 3,082,612 to Beckwith, which system draws ambient air into the main circulation path through ports located in the lower front panel of the refrigerated display cabinet. Such ports are normally closed during the refrigeration cycle and opened during the defrost cycle. The Beckwith et al, 3,850,003 patent indicates that the concepts described in Patent Nos. 3,082,612 and 3,402,525 did not prove to be practical and hence were not commercially feasible.
Some of the above-noted patents directed to air defrost systems use reverse air flow, during the defrost cycle of operation. U.S. Pat. No. 4,026,121 to Aokage, et al, which illustrates an open-front display case, and U.S. Pat. No. 4,120,174 to Johnston, which illustrates an open-top display case, also disclose reverse ambient air flows for defrosting. In these, ambient air can easily be drawn through the access opening of the case or cabinet into the air conduit through the outlet opening of the air conduit and then expelled from the air conduit after the defrost operation through the unblocked access opening. Such an arrangment, however, can not be readily used in a refrigerated merchandiser display case having barrier doors, since the front opening in the cabinet is covered by the doors. Thus, in order to employ an ambient air defrost system, a different type of system had to be conceived.
In seeking to employ ambient air defrost techniques in cases having doors, systems have been developed for drawing air through a limited portion of the air conduit by opening flaps into the conduit. These flaps are arranged so as to straddle the evaporator coils of the refrigeration mechanism. Such systems are disclosed in U.S. Pat. No. 3,226,945 to Spencer and U.S. Pat. No. 4,072,488 to Johnston. The patent to Spencer illustrates a plurality of different embodiments of open-top refrigerated display cases, both of the single shelf and multi-shelf types, in which the air flow is always drawn over evaporator coils in a single direction under negative pressure. During the refrigeration cycle of operation, air after being refrigerated is circulated through the air conduit and into the display section of the case. Pat. No. 4,072,488 to Johnston discloses a glass door type merchandiser display cabinet in which air is circulated through the air conduit and through the evaporator coils arranged within the air conduit in such a direction that cold air enters the display space at the bottom of the cabinet and is then drawn up into the air inlet located near the top of the cabinet. For defrosting, top flaps are opened since this case is designed with coils at the top. This shows a somewhat complicated way to provide both glass doors and air defrost features according to the prior art. Such systems are relatively complex and can involve certain operational problems, particularly due to frost and dust accumulation. Where there are moving parts inside of the air conduit an accumulation of frost on such parts can cause them to stick and hence not function properly.